The present invention relates to a magnetic field generator, and more specifically to a permanent-magnet-type magnetic field generator to be used for bending magnets, particle accelerators, magnetic resonance imaging apparatuses, cancer treatment apparatuses, cancer diagnosing apparatuses, chemical element analyzers, and so on.
Conventionally, for generation of a strong magnetic field by using permanent magnets, proposals are made for a magnetic circuit using a Halbach array, such as one disclosed in the Japanese Patent Laid-Open No. 2000-243621, in which generally triangular permanent magnets are disposed annularly.
With such a magnetic circuit, there was a belief that it is difficult to obtain a magnetic field intensity greater than an intrinsic coercive force of the permanent magnets, in its magnetic field generation space. Also, the permanent magnets normally used have an intrinsic coercive force of approximately 2700 kA/m, and therefore it has been difficult for the permanent-magnet-type magnetic circuit to achieve a magnetic field intensity not lower than 3 T in the magnetic field generation space.
In recent years, a strong magnetic field not weaker than 3 T is in demand for use in particle accelerators and other apparatuses, and the demand is currently met by superconductive electromagnets. However, these systems call for liquefied helium and a freezer therefore, leading to problems such as complexity in operation and increase in running cost.
It is therefore a primary object of the present invention to provide a permanent-magnet-type magnetic field generator capable of generating a strong magnetic field not weaker than 3 T in the magnetic field generation space.
According to an aspect of the present invention, there is provided a magnetic field generator comprising: a first magnetic circuit including first permanent magnets disposed annularly; and a second magnetic circuit including a pair of pole pieces provided inside the first magnetic circuit, and second permanent magnets for magnetic connection of the pole pieces, wherein the pole pieces are opposed to each other for formation of a magnetic field generation space, each of the pole pieces having a saturation magnetism not smaller than 1.6 T.
According to this invention, the magnetic flux generated by the first magnetic circuit can be further concentrated between the pair of pole pieces by the second magnetic circuit, and therefore it becomes possible to generate a strong magnetic field far beyond the intrinsic coercive force of the permanent magnets, or more specifically a magnetic field of not weaker than 3 T, in the magnetic field generation space. Further, use of permanent magnets in place of electromagnets enables to reduce running cost.
Preferably, a direction of magnetization of each second permanent magnet is not a 180-degree angle with respect to a direction of magnetic field of the magnetic field generation space. In this case, the direction of magnetization of the second permanent magnet is not truly away from the direction of magnetic field of the magnetic field generation space. Therefore, a demagnetizing field affecting the second permanent magnet is small, and the second permanent magnets are less prone to demagnetization by the magnetic field in the magnetic field generation space. This results in generation of a strong magnetic field in the magnetic field generation space.
Further, preferably, the second permanent magnets have a greater coercive force than the first permanent magnets. In this case, the second permanent magnets are less prone to demagnetization by the magnetic field of the magnetic field generation space. Therefore, there is even less demagnetization of the second permanent magnets, enabling to generate a strong magnetic field in the magnetic field generation space.
Further, preferably, the magnetic field generator further comprises third permanent magnets disposed on both end sides of the first permanent magnets. By providing the third permanent magnets on each end side of the first permanent magnets thereby sandwiching the first permanent magnets with the third permanent magnets, it becomes possible to decrease leakage flux. Therefore, it becomes possible to generate a strong magnetic field in the magnetic field generation space and to prevent formation of a strong magnetic field outside the magnetic field generator.